1. Field of the Invention
The present invention relates to devices for testing batteries, such as automotive batteries, and the charging systems therefor. The invention relates in particular to devices for testing the output voltage of a battery and the output current of an alternator for charging the battery, when the battery is under loaded conditions.
2. Description of the Prior Art
One of the standard tests for an automotive battery is a test of its capacity by monitoring its output voltage when it is under a load. Such load testing of a battery involves loading of the battery to a specified load current (such as one-half the rated cold cranking amperage) for a given period of time (such as about fifteen seconds), while monitoring the battery output voltage. If it exceeds a predetermined minimum voltage, the battery capacity is deemed adequate. Loading is accomplished by installing a variable impedance element, such as a carbon pile, across the battery. The electrical load resistance of the carbon pile is adjusted by manually turning a knob which is coupled to a threaded shaft which squeezes the carbon discs of the pile between a pair of brass plates. These brass plates are wired to the battery under test by means of heavy cables. Squeezing the carbon discs reduces the resistance between the discs, thereby increasing the load current through the battery.
Battery voltage is measured across the battery terminals using a voltmeter. Load current can be measured by scaling the voltage drop across a low-resistance, low-temperature-coefficient, series resistor or by using a nonintrusive, inductive probe with associated amplifiers. The user monitors the battery voltage while loading the battery with the carbon pile so as to maintain the specified load current.
The temperature within the carbon pile rises quickly as heavy current passes through it. This causes the resistance of the carbon pile to further decrease, thereby further increasing the load current. This is partly because carbon has a negative temperature coefficient of resistance, and also because the carbon discs may expand slightly with increased temperature, thereby effectively increasing the compression of the pile. The manual load control must, therefore, be continuously readjusted to maintain the load at about the predetermined current level throughout the test period, which is sometimes difficult to achieve. This difficulty is aggravated by the fact that the manual load control is biased to the zero load condition so that a high load will not inadvertently be left on the battery.
Loading of the battery is also involved in testing the alternator current output. For this test alternator current is measured directly by a suitable probe, such as a Hall effect current measuring probe encircling the alternator output cable. The automobile engine is operated at a moderate speed sufficient to ensure maximum alternator output, and then the battery load is varied with the carbon pile, while the operator monitors the alternator current output, taking note of and remembering the maximum current reading. If the maximum output current is sufficiently close to (e.g., within 10% of) the rated output, then the alternator is considered good.
It will be appreciated that this test operation is, of necessity, a two-person job or, at best, a difficult one-person job. Thus, one hand is required to hold the engine throttle to maintain a desired engine speed, another hand is required to manipulate the carbon pile, and then both battery output voltage and alternator current have to be simultaneously monitored. Most manufacturers specify a loaded battery output voltage at which the maximum alternator output should be achieved, so the operator must manipulate the carbon pile knob to attempt to maintain that load voltage. While he watches a voltage monitor to be sure that the proper voltage range is maintained, he must also constantly monitor an alternator output current display so that he does not miss the peak output current value.